Fuel and lubricant pumps for gas turbine engines have typically been offset from the centerline of the engine and driven, along with other accessories, by a gear train connected to the main shaft. See, for example, U.S. Pat. Nos. 3,521,505 and 3,576,375.
Due to the ever increasing number and complexity of engine accessories and the corresponding reduction of available space in the area of the accessory drive mechanism, it would be very desirable to locate such pumps within the main engine core.
Such an approach would not only conserve space but also eliminate the expense and weight of the gear train.
Several approaches have been tried in other types of turbomachinery to achieve similar benefits but the unique characteristics of high performance gas turbine engines prohibit easy adaptations of such prior designs. For example, large (and relatively crude) centrifugal turbopumps have been used to supply large quantities of fuel to the afterburner portion of military turbine engines and to liquid fueled rocket engines. See, for example, U.S. Pat. Nos. 2,606,501; 2,689,528; 2,778,312 and 2,956,502.
High speed, forced vortex pumps have been known since at least the early 1940's when they were investigated by Dr. U. M. Barske for use in rocket propulsion systems but they have not found wide commercial use since, probably because of their unfamiliar pumping characteristics. Physically, they somewhat resemble the common centrifugal pump but they operate on altogether different principles. A centrifugal pump uses a screw-shaped or scrolled impeller to force all the fluid which enters the pump to be thrown outwardly into an annular discharge channel. Since the fluid moves quickly through the pump, the residence time for any particular portion of the fluid is very short, often less than one revolution of the impeller, thus there is a considerable difference in relative speed between the fluid and the impeller. The characteristics of such pumps are generally well-known and they are commonly used to supply very large flows of fluid at low to moderate pressures.
In contrast, a forced vortex pump (not to be confused with a liquid-ring pump) is based on rapidly rotating a body of fluid and withdrawing only a relatively small portion of the fluid so that the remainder may be considered, for design purposes, almost as a rotating solid body. In its original form, such a pump consisted of a rotating drum with baffles or blades fixed to its inside walls for developing the rotating body. Fluid entered the drum through its hub and was picked up near its periphery by a stationary, internal pickup tube which exited the drum through the hub. Difficulties with adapting this design for various applications led to an inverted design in which a simple, straight impeller with long blades was used to create a rapidly rotating fluid vortex within a short cylindrical cavity within a fixed housing surrounding the rotating impeller. The outer portion of the fluid vortex adjacent the smooth housing wall, is at a high pressure while the inner portion is at a much lower pressure. Typically, the high pressure fluid is extracted from the housing through a tangential diffuser section where much of the kinetic energy (velocity) of the fluid is converted to static or potential energy (pressure). The pressure level at the discharge is determined by the diameter and rotational speed of the impeller, while the maximum output flow rate is directly related to the size of the diffuser throat at any given rotational speed. Very small, simple pumps can put out moderate flows at high pressure if all the components are carefully designed. In addition, these pumps can operate satisfactorily at very low input pressures, close to the vapor pressure of the fluid, without cavitation. More importantly, the output pressure is practically constant for all rates of flow at any given speed and the output capacity is approximately proportional to the impeller speed (up to a maximum value determined by the number and size of discharge).
However, such pumps are not readily adaptable to the physical limitations within the core of a gas turbine engine.
In view of the foregoing, it should be apparent that there is a need in the art for improvements in the construction and operation of fuel pumping and supply systems for small high performance gas turbine engines.
It is therefore an object of the present invention to provide improved methods and apparatus for supplying fuel to a gas turbine engine.
A further object of this invention is to provide a simple, highly reliable but low weight fuel supply system based on a modified vortex pump.
It is another object of the invention to provide a pump design which may be mounted in the core of a gas turbine engine over or around the main shaft which connects the compressor to the turbine.